The present invention relates to ultrasonic inspection, more particularly to a wide bandwidth ultrasonic transducer, and more particularly to a high energy, low frequency, ultrasonic transducer for acoustic inspection of long members, such as steel reinforcement rods located in concrete.
The ultrasonic sandwich transducer was developed 75 years ago by Langevin. Conventional ultrasonic systems are designed for the inspection of relatively thin sections with relatively unrestricted lateral dimensions. These ultrasonic systems are designed to perform inspections on materials with .gtoreq.3:1 width to thickness aspect ratio. This aspect ratio is required to avoid problems in waveform definition, such as refraction, reflection, dispersion, and diffraction that may distort the waveform. There are materials that do not and can not conform to this aspect ratio, but must still be inspected, such as rods, columns, cables, and tendons. Cylindrical structural members are used to strengthen concrete, and these steel reinforcements are often located deeply within concrete or geological structures. In all installations, the diameter of the reinforcements is much smaller than the length; as an example, 1.375 inch diameter by 50 feet long.
Attenuation of sound is proportional to the frequency of the sound and to the distance of sound propagation, among other factors. High energy ultrasound has had application in soldering, welding, and cutting of various difficult materials, but these instruments gain their advantage by the use of a modified cone, acoustic transformer that must be lightly coupled to the work surface and are unsuitable, due to damping, for the hard coupled propagation of sound into materials, such as a rod. In addition, the horn type acoustic transformer is generally excited with a continuous wave, and an inspection transducer is generally excited with a pulsed wave for the interrogation of echoes from defects. Thus there has been a need in the art for an ultrasonic inspection system for long length rods, columns, cables, and tendons.
The present invention provides a solution to the above-mentioned need by providing a wide bandwidth, ultrasonic transducer to generate nondispersive, extensional, pulsed acoustic pressure waves into concrete reinforcement rods and tendons. Two major problems were solved by the ultrasonic inspection transducer for rods, these being: (1) the ultrasonic wavelength must be greater than five times the major diameter of the rod, and (2) the bandwidth must be equal to or greater than the central operating frequency of the transducer to produce echoes that retain polarity and definition relating to the changes in rod dimensions along the length of the rod. Thus the present invention provides a high energy, low frequency, ultrasonic transducer, wherein the transducer acoustic impedance is matched to the rod impedance for maximum transfer of acoustic energy, and ultrasonic reflections will occur at points along the rod where there are changes of one percent of a wavelength in the rod diameter.